1. Field of the Invention
This invention relates generally to a tuning apparatus, for example, of a broadcast receiver, and more particularly is directed to an improved tuning apparatus of the type including a frequency synthesizer with a phase locked loop.
2. Description of the Prior Art
Existing FM tuners usually employ a variable condenser actuated by manipulation of a tuning knob for varying the local frequency of a local oscillator so as to select the radio wave broadcast by a desired station for reception by the broadcast receiver. Manipulation of the tuning knob also causes relative movement of an indicator and dial provided with a scale calibrated to represent broadcast frequencies over a predetermined frequency band. If the user knows the assigned frequency of the desired station or channel, the tuning knob is manipulated until the indicator is disposed at such assigned frequency on the cooperating scale and the variable condenser then more or less achieves the local frequency for receiving the radio wave broadcast by the selected or desired station. If the user does not know the assigned frequency of the desired station or channel, then the tuning knob is simply manipulated to cause the indicator to scan the scale and to similarly actuate the variable condenser until the desired station is being received. In either case, accurate tuning is not always possible by reason of the fact that the variable condenser is being manually actuated or controlled, and the foregoing problem is not fully avoided by providing the receiver with a tuning meter.
In order to provide more accurate tuning of broadcast receivers, an AM and FM receiver has been proposed that uses a frequency synthesizer with a phase locked loop, for example, as described in Fairchild Semiconductor's application by J. Stinehelfer and J. Nichols, 1969, entitled "A digital Frequency Synthesizer for an AM and FM Receiver". Such frequency synthesizer for tuning an FM and AM radio essentially consists of a voltage-controlled oscillator, a programmable divider, a frequency and phase comparator, and a reference frequency generator. The voltage-controlled oscillator is the local oscillator of the tuner, and the output signal of the voltagecontrolled oscillator is divided by the programmable divider, whereupon the frequency and phase of the signal thus divided is compared, in the comparator, with the crystal-controlled reference frequency or signal. The resulting voltage output or control signal from the comparator controls the voltage-controlled oscillator so that the local frequency of the latter will be a selected multiple of the reference frequency, as determined by the dividing ratio of the programmable or variable frequency divider.
In the above frequency synthesizer, the programmable or variable frequency divider may be constituted by a down counter which is operated by clock pulses corresponding to the local frequency signal, usually after passage of the latter from the voltage-controlled oscillator through a prescaler or fixed divider. The down counter is loaded or preset with the value of a selected dividing ratio each time the counter has counted down to a predetermined value, for example, to "0", at which time a pulse is also applied from the variable frequency divider to the comparator. After such loading or presetting of the counter, each clock pulse merely causes the counter to count down by one until it has again counted down to the predetermined value to emit a pulse and to be again preset with the value of the selected dividing ratio. It will be apparent that, with the described frequency sunthesizer, a different dividing ratio has to be provided for each of the broadcasting stations to be received.
The FM broadcast band in the United States consists of one hundred channels, each 200 KHz wide, starting at 88.0 MHz. The carrier for the first channel is at 88.1 MHz, and the carrier for the last channel is at 107.9 MHz. By reason of the standard 10.7 MHz intermediate frequency that is employed, the local frequencies to be produced by the voltage controlled oscillator have to be 10.7 MHz higher than the frequencies of the carriers. Thus, the local oscillator frequencies that are required range from 98.8 MHz to 118.6 MHz, with 200 KHz spacing between adjacent local oscillator frequencies. In any case, it will be apparent that in an FM broadcast receiver having a digital frequency synthesizer for use in the United States, provision has to be made for selectively loading or presetting one hundred different or discrete values of the dividing ratio in the variable frequency divider, which values respectively correspond to the one hundred channels of the FM broadcast band. In existing broadcast receivers, as aforesaid, station selection has been achieved by providing the tuning apparatus with a panel having an array of push-buttons respectively corresponding to the various stations or channels of the FM broadcast band, with each push-button being effective, when manually depressed, to cause an associated control circuit to apply a respective control signal to the variable frequency divider for establishing the desired dividing ratio in the latter. However, with such a station selecting arrangement, if the user does not know the assigned frequency of a desired station or channel, the successive actuation of the push-buttons for locating such station is a bothersome and time consuming operation, particularly as many of the push-buttons will correspond to stations of the broadcast band that are not receivable at the location of the broadcast receiver. Accordingly, it has been proposed, for example, as disclosed in detail in U.S. Pat. No. 3,845,394 issued Oct. 29, 1974, and having a common assignee herewith, to provide the described type of tuning apparatus with circuit arrangements by which the broadcast band may be searched to identify and memorize those stations from which the transmissions can be received and then to scan the receivable stations in succession or in a step-by-step manner. However, such circuit arrangements are quite complex and costly.
It has further been proposed, for example, in U.S. Pat. No. 3,163,823, issued Dec. 29, 1964, to provide the described type of tuning apparatus with a station selector that includes at least three switches respectively corresponding to the tens, units and tenths of the broadcast frequencies and which are individually actuated by the turning of respective thumb wheels through ten suitably labelled and detented positions for closing and opening respective sets of contacts by which the dividing ratio of the variable frequency divider is selected. It will be apparent that, with the foregoing station selector, scanning of the broadcast band to locate a desired station whose assigned frequency is not known requires the repeated successive actuation of the three thumb wheels, and this is again a bothersome and time consuming operation.